1. Field of the Invention
The present invention relates to rocket motors, especially for gun-launched projectiles, having insensitive munitions systems.
2. State of the Art
Many launchable projectiles, rockets, and rocket stages comprise a forward end, including guidance and munitions, and an aft end rocket motor. These two elements can be formed together, with a common outer case, or they can be separately formed and subsequently joined together. This joining can occur immediately prior to use, in which case the two elements may be separately stored, or the elements can be joined together for storage purposes and be ready for immediate use.
During prelaunch storage, when a rocket motor is ignited inadvertently by external heating, such as a spilled fuel fire, the rocket motor may become propulsive before being properly aimed. When inadvertent ignition is caused by fragment impact that produces unplanned nozzle outlets, the motor may become wildly propulsive in undesired directions. When such events produce unplanned increases of propellant burning surface area, excessive pressurization may increase the hazard to nearby personnel and property. In light of these dangers, many of today's weapon systems must satisfy certain insensitive munitions (IM) requirements focused on safe storage capabilities.
One way that rocket motors meet IM requirements is by venting the internal pressure caused through inadvertent ignition of the propellant by discharging either the forward or aft closure of the case cylinder. This allows the propellant to burn through a now open end that is relatively large compared to the nozzle throat without generating substantial thrust in any direction and without the threat of the rocket motor exploding and spraying burning propellant and metal case cylinder fragments in numerous directions.
The prior art teaches the use of dual paths for load transfer between features of either closure or between the closure and the motor case cylinder. One such load path may be sized to accommodate relatively small loads that might be experienced during transportation and handling prior to gun launch, and the other to accommodate much larger loads encountered during launch or during rocket motor operation. Focusing on shells that may or may not include rocket motors, U.S. Pat. No. 4,557,198 discloses shear pins or locking rings arranged for arming the high capability load path by axial acceleration during normal launch and disarming the low capability load path. Boissiere, in U.S. Pat. No. 5,337,672 (1994), teaches arming of the high capability load path and disarming the low capability load path by gas pressures produced by the round itself. Dolan, in U.S. Pat. No. 4,597,261 (1986), Panella in U.S. Pat. No. 3,887,991 (1975), Tate in U.S. Pat. No. 5,036,658, Koontz in U.S. Pat. No. 5,155,298 (1992), Ellingsen in U.S. Pat. No. 5,311,820, and Cherry, in Statutory Invention Registration H1144 disclose the use of thermally activated devices of similar intent. Further, Malamas, in U.S. Pat. No. 4,991,513, discloses use of a vent system that is closed by spin-up at launch. Singer et al., in U.S. Pat. No. 6,094,906, discloses a more recent approach for generating a vent path for IM protection.
The safe expulsion of either closure can also be accomplished through the use of a low shear retaining means—positioned between components of the closure or between the closure and the rocket motor case cylinder—and a high capability load path that is disarmed until subjected to gun pressure. Should the propellant be inadvertently ignited, the low shear retention means will shear under relatively low internal pressure and allow the entire closure, or a portion thereof, to disengage from the case cylinder. Thus, the internal pressure induced by inadvertent ignition will vent without the dangers associated with premature propulsion or explosion.
One problem associated with many of these conventional IM systems is that they do not pass slow cook-off tests. For many conventional IM systems, heating at relatively slow rates of about 6° F./hr causes the entire propellant to combust substantially instantaneously prior to activation of the IM systems, producing excess gas which the IM systems are not equipped to handle and safely expel.
In the case of gun-launched missiles, other design criteria that should be taken into consideration pertain to the thermal expansion characteristic of composite solid propellants. Composite solid propellants are one of two general types of solid propellants for rockets. In composite solid propellants, the fuel and oxidizer particles are bound together by a cured rubber matrix. Composite propellants have burning surface areas that may be readily controlled by adjusting the shape of the solid material and the burn rate features of the formulation. The other type of solid propellant is compressed powders. For compressed powders, virtually the entire cumulative surface area of all the particles is available for combustion immediately upon ignition. During the burn of a compressed powder propellant, vastly higher operating pressures prevail than during burn of a like quantity of composite propellants. It follows that compressed powder propellants are generally used only where the gun barrel can withstand the high pressures. When the propellant is to burn after the rocket leaves the gun, generally a composite propellant is chosen.
Typically, a composite solid propellant has a thermal expansion characteristic that is an order of magnitude larger than that of the enclosing or containing structure. A 100° F. (56° C.) change in operating temperature therefore may produce a propellant volume change of about 2%. Unless the configuration and support arrangement allow deformations to occur, thermal stresses in the propellant may cause fractures, undesired increases of burning surface area, and disasters upon ignition. Common provisions for thermal expansion include a central axial perforation for propellant grains bonded on their outer circumferential surfaces to cylindrical vessels and completely free outer surfaces for propellant grains bonded at either their forward or aft ends to vessel closure features.
The threat that gun accelerations may pose to the integrity of a propellant charge may be great unless care is exercised over the propellant configuration and means of supporting the propellant. Accelerations imposed within the gun tube upon gun-launched projectiles are hundreds—even thousands—of times larger than those for rocket-launched projectiles. The tensile and shear strengths and elastic moduli of typical propellants are minuscule in comparison to the containing structure. For this reason, departures from a hydrostatic stress state during gun launch are accompanied by large deformations. At high forward acceleration, the propellant grain tends to completely fill the available volume of the aft end of the containing vessel.
During gun launch, alternatives to the aft end support arrangement for the propellant grain can be grave threats to the integrity of the propellant grain. Indeed, at acceleration levels typical of gun launches, neither the bonded circumferential surface of an axially perforated propellant grain nor an unperforated grain with a bonded forward end is stiff enough to eliminate the aft end support mode unless there is a great deal of empty space within the motor.
It follows that virtually the entire force that accelerates the propellant grain during gun launch is applied by direct bearing through its aft end. It also follows that the circumferential surface of the propellant grain will expand to fill the cylinder, imposing a radial pressure varying with depth (hydrostatically) from the aft end to the forward end.
Therefore, during gun launch, the case cylinder usually experiences tension in the hoop direction due to internal pressure applied by the propellant. This internal pressure may well be several times larger than the operating pressure later in flight when the propellant burns. Moreover, during gun launch, the axial force needed to accelerate the payload located forward of the rocket motor is carried around the propellant grain by axial compression in the rocket motor case, which should be proportioned so that buckling does not occur.
The buckling load for an axially compressed thin cylinder depends on its radius, thickness and length, and upon the modulus of elasticity at the actual imposed effective stress level. When the material “yields,” the modulus decreases from the initial value, Young's modulus, to zero eventually (for ductile metals). Effectively, the material yields under the mixed tension and compression condition at a far lower stress level than if either stress were acting alone, and the modulus of elasticity—and the buckling load—are thereafter much reduced. Thus, the thickness needed to assure a suitable safety factor is expected to be much higher than would be deduced for either the internal pressure or axial force alone.
In recent years, efforts to overcome the above-described behaviors of both the propellant and the case cylinder have turned to admitting the gun pressure to the interior of the rocket motor case. Examples of this approach are disclosed in U.S. Pat. No. 3,349,708, and are also explained in detail in U.S. Pat. No. 6,094,906. Admitting the gun pressure to the interior of the rocket motor with the fluid void-filler has both obvious and subtle implications. Among the obvious is that unless the exterior surface of the rocket motor is also exposed to gun pressure, the case cylinder may have to accommodate as much as 60,000 psi internal pressure, or more—an order of magnitude above the usual range of rocket motor operating pressures. To expose the external surface to pressure, an obturator, which is a sliding seal between the projectile and the gun tube that prevents the gun pressure from escaping around the projectile, is moved from the aft to the forward end of the rocket motor. It follows that, for the quasi-static situation at maximum acceleration, the differential pressure across the case cylinder wall is external pressure of varying magnitude, reflecting the hydrostatic gradient in the propellant grain. Further, the axial compression in the case cylinder disappears because the accelerating force for the payload is applied directly to the forward closure.
The subtle implications reflect the dynamic situations as the gun pressure rises rapidly upon ignition and as the gun pressure disappears when the obturator passes out of the gun bore. At the outside, because the orifice into the rocket motor is quite small, the intensity of the gun pressure applied to its interior lags the pressure intensity applied to the exterior. This threatens to buckle the case if the duration of the lag is large enough. Also when the obturator clears the gun bore, the small nozzle orifice prevents an instantaneous drop of internal pressure after the external pressure disappears. This threatens to burst the case unless it has been made thick enough to withstand the gun pressure level—acting alone—that prevails immediately before the obturator clears the gun bore.
Given usual propellants and rocket motor nozzles, greater range or velocity is achieved for the projectile by configuring the rocket motor such that it can hold a maximum amount of propellant. However, the outside diameter, and hence the available volume for propellant, of gun-launched rocket motors is limited by the size of the gun bore from which the rocket motors are fired.
The volume of propellant in gun-launched rocket motors is maximized when the interior diameter of the rocket motor case cylinder is maximized by making the case cylinder as thin as possible. However, the case cylinder should be designed thick enough to withstand gun-launched loads and, when gun pressure is allowed within the case cylinder, the pressure differentials between the inside and outside of the case cylinder. The case cylinder should further be designed to withstand pressure differentials not only at maximum levels, but as the gun pressure rises early during launch and falls as the rocket motor exits the gun bore. Rocket motors designed according to the prior art must therefore survive gun launch loadings that are frequently far more severe than the later loadings during rocket motor bum. This may require thicker structures which diminish the volume available for propellant, and which increase the inert weight of the motor, thereby diminishing the attainable range or velocity of the projectile.
Thus, an advancement over the prior art would be achieved by introducing rocket motor configuration features that diminish the net loads that the rocket motor case cylinder must be designed to withstand during gun launch, thereby diminishing the inert weight and increasing the available propellant volume while also providing an insensitive munitions system that is effective against slow cook-off conditions.
Such rocket motor configuration features are disclosed and claimed herein.